First of all, Batman didn't have to make a choice between one life or the other. Dent doesn't have to die for Bats to tackle and disarm him, it's an accident.

Second, how is his choice utilitarian? It was based entirely on the fact that Harvey had done some terrible things, and Gordon's son was completely innocent.

Finally, what does "he made the choice nobody else could make; the right choice" mean? You think Jim Gordon or any other cop for that matter wouldn't have been capable of making the choice to save an innocent kid versus the man holding a gun to him?

1. Yes, it was unlikely his intent to kill Dent. He saved the kid, and as a result, Dent fell and died, like he himself could have done (died).

2. It is utilitarian because Dent was on a rampage, murdering the guilty and innocent alike. Not allowing him to continue, regardless of the means, results in a positive net utility to society; take one life to save countless. It's very similar to altruistic behavior; sacrificing yourself to save many. I don't think it was Batman's intent to kill Dent, but we will never know, unless they address it in DKR which I highly doubt would happen conclusively. He is afterall quite well versed in disabling but not killing people, maybe it was a mistake. But it resulted in positive utility regardless of intent; who knows how many murders he prevented? At least one that we know of, probably three in the immediate future...

3. Why so serious? I'm just spamming Batman quotes, just like I did with the "killing is making a choice..." bit. They're only slightly relevant